UV-Vis

what is spectroscopy?

study of interaction btwn electromagnetic (EM) radiation and sample.

interaction is based on EM rad. being absorbed or emitted by sample

what is spectrometry

measurement of EM rad. absorbed or emitted by sample

what is electromagnetic radiation + two representations

• a form of light energy transmitted through space at high velocities

• a stream of particles carrying energy, called photons. each travelling in a wave-like pattern at the speed of light

rep as wave model/particle (photon) model

what is wavelength λ (m)

linear distance of one complete cycle btwn successive maxima/minima

what is frequency v (Hz)

no. of waves passing a fixed point per unit time/ no. of oscillations that occur in 1s

equation for wave model

c = vλ

c - speed of light, v - freq, λ - wavelength

equation for particle model

$$E=hv=\frac{hc}{\lambda}$$

$6.63\cdot10^{-34}$ s → h / Planck’s constant

name 2 types of spectroscopy

absorption spectroscopy

emission spectroscopy

when does absorption of energy occur (ground to excited state)

• electrons in atoms/ions/molecules usually present in GROUND state

• if a photon passes near the e-, energy of photon matches energy difference btwn ground & excited state

• energy from photon is transferred to electron → EXCITED state

in this spectroscopy, energy states & amt of energy that can be ____ are discrete

absorption spectroscopy

what is the name of the plotted graph for radiation absorbed by sample & how does it differ for atoms and molecules

absorption spectrum. atoms are straight vertical lines while molecules is a graph line joined together

what is the UV-Vis absorption range

Near-UV region: 180-380nm

Visible region: 380-780nm

*vacuum UV region: 10-180nm (not useful bc air absorbs most of EM rad. in this region & instruments need to under vacuum conditions)

how does absorption of radiation affect how we see coloured objects

• When white light passes through an object, some wavelengths of light will be absorbed

• unabsorbed wavelengths of light are transmitted

• residual transmitted wavelengths are seen as colour (COMPLEMENTARY to what is absorbed!!!)

what are chromophores and chromogens

chromophores - the absorbing groups in chem. species

chromogens - chem species containing chromophores

basically: chromogens have chromophores

examples of chromogens (organic & inorganic species)

organic:

• saturated compounds with N, O, halogens

• unsaturated compounds with double, triple bonds, aromatic rings

inorganic:

• transition ions with PARTIALLY filled d-orbitals

• ions that can form soluble complexes with chelating reagents

equation for absorbance of radiation

$$A=\log_{}\left(\frac{1}{T}\right)=\varepsilon bc$$

A - absorbance, b - optical pathlength (cm), ε - molar absorptivity (M^-1cm^-1), c - concentration (M or mol/L), T - transmittance

what is transmittance (T)

ratio of radiant power (I) in a beam of radiation after it has passed through a smaple to the power of the incident beam (I_o)

what is Beer’s Law

The absorbance is directly proportional to pathlength and concentration of sample.

A ∝ b,c

what are the 2 factors that contribute to deviations from Beer’s Law

Fundamental limitations of Beer’s law

• at high conc (>0.1M) indiv atoms/molcules no longer behave independently of o/a

• due to proximity, interactions of atoms occur & affects ability to absorb rad. → change in ε

Instrumental deviations

• output from continuous light source will always produce a specific band width (±5nm)

• always results in negative deviation

Beer’s Law is only valid for low concentrations of analyte and monochromatic radiation.

What are the 4 main components for optical spectroscopy

light source, monochromator (wavelength selector), sample chamber, detector

what are the qualities of a good light source and EXAMPLES of light source

qualities: (provide appropriate λ for analyte in sample to absorb at)

• beam emits rad. over a wide spectral range

• adequate intensity to be detected

• provides stable output

examples:

• tungsten filament lamp (Vis)

• Deuterium discharge lamp (UV)

what is a monochromator for

aka wavelength selector - to disperse light into its component λs, allow rad. with a narrow wavelength to pass through exit slit to sample

eg. prisms, reflection gratings

what is a sample chamber for & examples, some factors to consider when selecting sample cells

to contain sample, must be transparent in the λ region being measured

eg. quartz cuvette (UV, Vis), glass cuvette (Vis)

factors: λ of rad. used, amt of sample avail., nature of sample (aq or organic)

what does a detector do and how does it work

Record and convert electronic signal to instrumental signal

• photomultiplier tubes for signal amp

• tube contains photo-emissive cathode & several dynodes in vacuum

• cathode coated w/ easily ionizable material eg. alloys of alkali metals (K, Na, Ca, Mg) w/ Sb, Bi, Ag

single beam vs double beam

single beam: sample and reference cells are read at different times

double beam: monochromatic beam split into 2 components, one beam for sample & other beam for reference

• compensate fluctuations & wavelength changes from rad. source

• allows continuous recording of spectra (absorbance as a function of λ)

how to find conc of unk. analyte?

conc must be within linear dynamic range of the series of standard solutions prepared (at a range of appropriate conc) eg. 5 standards

standard curve method & standard addition method can be used

what is matrix interferences & 2 ways to eliminate it

when any other chem. component in sample absorb in the same wavelength as analyte of interest

• blank correction

• standard addition method

how does blank correction work (to elim. matrix interference)

measure a blank (0 conc of analyte) against the sample to find conc of analyte

note: hard to obtain a representative blank (need identical sample w/o analyte) so can cause inaccuracies in test result

how to use standard addition method

prep series of analyte standard solutions w/ same amt of sample added into each

measure each standard on UV-Vis

plot absorbance against concentration

calculate analyte conc using y = 0